(1) Field of the Invention
This invention relates to a variable equalizer. More particularly, it relates to a variable equalizer which is incorporated in a transmission line in order to compensate for a fluctuation in the frequency characteristic of a cable for communication and to guarantee the transmission characteristic thereof.
(2) Description of the Prior Art
As variable equalizers to be used in transmission lines for communication, there have heretofore been known many types complying with required frequency characteristics and intended uses. The inventors have proposed variable equalizers employing no inductance element which have comparatively simple circuit arrangements and which can be easily fabricated as integrated circuits (IC's) (U.S. Pat. No. 4,004,253 and U.S. Pat. No. 4,080,580). The variable equalizers according to these inventions comprise in combination a circuit which has a frequency characteristic Y(f) dependent upon the frequency f, and a variable amplifier which is independent of the frequency (and whose gain is denoted by x). The characteristic or transfer function v(f, x) of the variable equalizer is a linear equalization characteristic of: ##EQU1## or a quadratic equalization characteristic of: ##EQU2## Here, ##EQU3## Thus, when x varies from 0 (zero) to .infin. (infinity), the characteristic of the variable equalizer is varied from Y(f) to 1/Y(f).
The characteristic of a variable equalizer, however, differs depending on the sort of a communication cable used or the transmission distance. In order to realize a variable equalizer adapted for communication cables of various transmission distances, it is required that the frequency characteristic of the variable equalizer varies in a range of 1-1/Y(f).sup.2 in accordance with the variation of the gain x. In order to satisfy this requirement, a circuit having a frequency characteristic 1/Y(f) needs to be added to the output end of the variable equalizer having the frequency characteristic (1) or (2) as will be explained later with reference to the drawings. The circuits of the characteristics Y(f) and 1/Y(f) are ones determining the frequency characteristic, and their constituent elements must have a high precision. Therefore, the addition of the circuit usually leads to a high cost.